Cactus is a family of plants that are specially adapted to survive arid conditions, most often having leaves reduced to spines, and succulent characteristics. The scientific family name Cactaceae is applied to this group comprising 121 different genera. This plant family is concentrated in the Americas and has a surprisingly broad latitude range in both the Northern and Southern Hemispheres.

A considerable number of cacti species are threatened, chiefly due to habitat loss to agriculture, trampling by illegal human immigration into the southwestern USA, large-scale desert solar power projects, and overcollecting.

Distribution

The cactus family is generally considered native to the Americas. A notable exception is Mistletoe cactus, Rhipsalis baccifera, which is thought to have spread, fairly recently, from the American tropics to the subtropics and the deserts of the southwestern USA. Several species have been naturalized to the Mediterranean Basin, Australia and Hawaii.

There are three distinct centers of cactus biodiversity. The northern center is in Mexico and the arid southwestern USA, including the Sonoran and Mojave Deserts. Prominent tribes in this region are the Cacteae and Pachycereeae. Another center of diversity is located in the eastern portion of Brazil, embracing the campo rupestre region of steep rocky slopes and the dry caatinga (Tribe Cereae is a prominent taxon here.). The third center is defined by the arid and semi-arid parts of the southwest Andes Mountains, including portions of Chile, Argentina, Peru and Bolivia. Principal tribes in this region include Trichocereae, Notocacteae and Browningieae.

For cactus species that are tropical epiphytes there are two smaller centers of diversity. One is in Bolivia, where there is a high diversity of the Rhipsalideae tribe as well as the Hylocereeae.. A second center is within the Atlantic rainforest of southeastern Brazil.

The northernmost occurrence of the cactus family is Opuntia fragilis, found as far north as the Peace River in British Columbia and Alberta; the southernmost occurrence is Maihueniopsis darwinii, found in Argentine and Chilean Patagonia; other Southern Hemisphere extreme latitude species are Pterocactus australis and Maihuenia poeppigii. South American cacti occur as far west as Bartholomew Island in the Galapagos Islands, where Opuntia galapageia is found. Members of the family growing at high altitude include Austrocylindropuntia floccosa, which has been found at 4500 meters in the Andes.

The subfamily Opuntioideae is the widest ranging group of cacti, with many species commonly found in both North and South America. Genera that occur in tropical areas of both continents are Rhipsalis, Pilosocereus, Pereskia, Melocactus, Mammillaria, Hylocereus and Harrisia.

Morphology

The fundamentals of cactus morphology resemble those of most flowering plants. The great majority of cacti have leaves that are reduced to spines, which minimizes moisture loss and provides defense against herbivory. These spines emerge from a node-like structure called an areole. This cushion-like organ also supports either a flower or branching stem. Two exceptions are the genera Pereskia and Pereskiopsis, that have non-succulent leaves rather than spines. The first of these is closely related to the earliest cacti.

Evolution has caused the stems of many cacti to become thickened, resulting in an optimal spherical or cylindrical shape which minimizes water loss by providing a high ratio of volume to surface area. This also protects the plant against excessive sunlight. The stem surface manifests a waxy coating that aids in water retention; however, stems and spines can absorb moisture, which process is vital for species that receive most of their water intake via fog.

Stems engage in photosynthesis, using the mechanism of Crassulacean acid metabolism, a process whereby waste oxygen is not expelled until after dark, thus minimizing transpiration of water vapor.

Root structures are typically shallow, but highly branched in a lateral formation. In columnar cacti, there is also a taproot, an adaptation for seeking deep water. Roots have a high saline content, a feature that maximizes uptake rate from fresh rain.

Relation to Abiotic factors

Cacti are influenced by soil nutrient content, soil porosity, rainfall, pH and other factors. Due to the relatively high content of woody tissue in their stems, most cacti are quite resistant to forces of gravity and wind. In the columnar Carnegiea gigantea, these tissues stiffen with age, permitting it to reach heights of up to 15 meters. Similarly, the Saguaro cactus grows as tall as 18 meters, supported by stiff woody ribs.

Habitats

Dry Limestone Shrubwoods

This dense plant community is found on bare rocky limestone terraces, characteristically inboard of limestone pavement, as well as lowland karstic topography. The prevailing plant association is that of spiny, sclerophyllous small-leaved shrubs and trees. One example of this habitat type is found on some eastern Caribbeancoastal plain locations, involving semi-arid scrub forest on a karst limestone terrace. There are instances of dry limestone shrubwoods in northeasterrn Arizona on the Kaibab formations; in many cases this limestone substrate engenders a habitat for highly specialized cacti. For example on the Kaibab Plateau, the rare cactus, Pediocactus bradyi, is found, a species dependent on these limestone soil and rock terraces, which is not found elsewhere outside of Marble Canyon and Coconino County in Arizona.

Sonoran Desert

Sonoran Desert, Anza Borrego. @ C. Michael Hogan
The desert floor and bajadas (convergences of alluvial fans into a single apron of deposits below a slope) of the Sonoran Desert are the mildest of North American deserts. They are also the wettest, experiencing the only bi-annual rainfalls of any North American arid land. Consequently these habitats are also the ecologically richest and most biodiverse. Soils are coarse sands riddled with dry streambeds and washes. Dominant megaflora vary from Saguaro cactus to Organ pipe cactus and various Fouquiera species in the Mexican reaches. Besides hundreds of other Cactaceae species, there are a wide variety of trees and shrubs such as Creosote bush, Ironwood and Palo Verde tree. Riparian zones support such species as Velvet ash, Arizona sycamore and hopbush. Wildflower species include Paper flower, Desert marigold and Mexican poppy.

Mojave Desert

The Mojave Desert is cooler and drier than the Sonoran and also spans a wider elevational range, from below sea level in Death Valley to above 1600 meters. The chief megaflora is the cactus species Joshua Tree (Yucca brevifolia). The Mojave's cacti include species such as Silver cholla, Opuntia echinocarpa, Mojave prickly pear, O. erinacea, Beavertail cactus, O. basilaris, and Many-headed Barrel Cactus, Echinocactus polycephalus. Other associate species include Spiny menodora, Menodora spinescens, Desert senna, Cassia armata, Mojave dalea, Psorothamnus arborescens, and Goldenhead, Acamptopappus shockleyi. Soils are coarse sands and gravels with many outcrops that offer habitat niches. There are also several important oak species such as Quercus turbinella and Q. gambeli, which are found in moister lowlands and rocky outcrop areas.

Great Basin

Great Basin scene, southwest Utah. @ C. Michael Hogan
The Great Basin is a true cold desert and one of the poorest with respect to cactus species. Soils are derived from a gamut of rocks, including colluvial wash on mountain slopes and thick lacustrine and shore deposits associated with ancient Lake Bonneville. Joshua Trees are only patchily present as far north as southwestern Utah. Prominent woody plants of the Great Basin include Rabbitbrush, Chrysothamnus nauseosus, Utah juniper, Juniperus osteosperma, One-needle pinyon pine, Pinus monophylla and Two-needle pinyon pine, P.edulis.

Atacama Desert

Topography and soil substrate combine to influence the patterns of moisture availability, producing one of the most arid coastal deserts in the world, similar to the Namib Desert of Africa. Where isolated mountains or steep coastal slopes intercept the clouds, a fog zone develops with a stratus layer concentrated against the hillsides, allowing the development of fog-zone plant communities near the coast and in lower portions of numerous gorges at elevations below 1100 meters. These plant formations also have been called the fertile belt, fog oases or meadows on the desert. Plant communities here consist of mixtures of annual and short-lived perennial and woody scrub vegetation.

Ecuadorian Dry Forests

The Ecuadorian dry forests are an example of dry forests in the Central and South American region. These Ecuadorian dry forests recieve scant rainfall much of which is actually sea mist. This dry forest has trees that reach 20 meters high, and hosts abundant bromeliads, mosses and epiphytes, while the undercanopy has many species of thorny shrubs and cactus. The understory is dominated by small herbs mainly from the Acanthaceae and Polypodiopsidae families. The southwestern part of this ecoregion is influenced by the Humboldt Current.

Aruba-Curaçao-Bonaire cactus scrub

Being only about twelve degrees from the equator, this habitat is quite dry to to high evapotranspiration. Underlying soils are derived from limestone and quartz formations. Most of the natural areas on Aruba, Curaçao and Bonaire are sparsely vegetated with cactus scrub. Columnar cacti are common, and can grow up to six meters in height. The dominant cacti species are: Stenocereus griseus, Subpilocereus repandus, and Pilosocereus lanuginosus. These species flower and fruit profusely during the dry season, and provide critical resources for a variety of bats, birds and other animals. Also characteristic of the area is the wind-swept divi divi tree (Caesalpinia coriaria, a non-cactus scrub species in the fabaceae family).

Ecology

Cacti have important relationships with the fauna and other flora that share their habitats.Hundreds of species of ants use cacti for food. They also serve, along with other insects, as key cactus pollinators. Where other predators would find the spines daunting, ants forage along cactus stems, capturing small fauna and feeding from extrafloral nectaries. These nectaries provide a sugar mixture that is often high in amino acid content. Furthermore, ants and other insects feed on cactus seeds and play a role in seed dispersal.

The Bee assassin, Apiomerus crassipes, lies in wait in cacti flowers and preys upon its bee or ant victims by injecting a paralyzing enzyme with its hypodermic-like beak. The Cochineal insect, Dactylopius coccus, is a cryptic species (a taxon that uses anatomy or behavior to elude predators), the females of which spend their entire lives in colonies upon the stems of Prickly pear; this insect species is notable for the secretion of carminic acid, used by the ancient Aztecs to produce a crimson dye. Hundreds of butterfly, moth and skipper species are known pollinators of the Cactaceae family.

Larger cactus species, such as the Saguaro, are significant nest sites for birds, rodents and other animals. The Gila woodpecker is a particularly important architect, creating deep cavities in the Saguaro, which not only host the Gila, but also numerous other species in successive years.

Larger mammals such as Javalina and White-tailed deer actually consume the pads of prickly pears, in spite of the formidable spiny armor.

Evolution and Phylogeny

The sub-families of cactus differentiated about 70 million years ago. Approximately 50 million years before the present, Cenozoic cooling produced regionally arid conditions which further stimulated their evolution. A particular jungle thorn bush was among these early cacti, which bore all seven characteristics with which Cactaceae are endowed. This ancestral cactus led to descendants with regional adaptations. Pereskia aculeata, a robust woody shrub with succulent leaves which produces the Barbados Gooseberry, is believed to resemble the ancestor. Adaptation has produced highly succulent stem morphologies, with secondary xylem tissue of highly patchy cells lacking lignin, thus allowing water retention and more ready paths for diffusion from vessels into succulent tissues of the cortex and pith.

This ancestor was the first to evolve an areole structure as well as the specialized axilllary bud, distinctive to the Cactaceae family. Each axillary bud carries the spines and a bud which sometimes differentiates to produce either a flower or side shoot.

Cactaceae variation and biogeography poorly known due to the paucity of the fossil record. No fossil cacti are extant; however, DNA analysis of extant species provide considerable cladistic detail illuminating the evolutionary history of the Cactaceae. More than 2000 species of cacti have developed to the present time.

Taxonomy

Leuctnebergia principis, Cacteae tribe, whose species either have no leaves at all, or have stems that resemble leaves.
The following taxonomy includes the recognized subfamilies and tribes ot the family Cactaceae, but does not list any hybrid genera or synonyms.

Subfamily Cactoideae has the largest number of species of any subfamily and is broadly geographically distributed. Tribe Cacteae alone possesses three fourths of all the cactus species.

Subfamily Maihuenioideae is the smallest subfamily, having only two species, both of which are endemic to the high Andes of Chile and Argentina.

Subfamily Opuntoideae is a very diverse group, comprising the most cold tolerant of all cacti. They have an extremely broad geographic distribution as far south as the Galapagos Islands and north through Mexico and the western USA, as far north as british Columbia and as far northeast as the Great Lakes states.

Subfamily Pereskioideae typically occurs in tropical and subtropical dry broadleaf forests or xeric shrublands; these species are found from southern Mexico to Venezuela, as well as the entire Caribbean Basin and eastern Brazil southward to northern Argentina.

Subfamily Cactoideae

Stetonia coryne, only species in its genus. Source: Dave Pape

Tribe Browningieae

ArmatocereusBackeb.

BrowningiaCárdenas

Gymnocereus Rauh & Backeb.

JasminocereusBritton & Rose

NeoraimondiaBritton & Rose

StetsoniaBritton & Rose

Tribe Cacteae

AcharagmaN.P.Taylor & Glass

AriocarpusScheidw.

AstrophytumLem.

Aztekium Boed

CoryphanthaEngelm Lem.

Echinocactus Link & Otto

EchinomastusBritton & Rose

EpithelanthaF.A.C.Weber ex Britton & Rose

EscobariaBritton & Rose

FerocactusBritton & Rose

GeohintoniaGlass & W.A.Fitz Maur

LeuchtenbergiaHook.

LophophoraJ.M.Coult

Mammillaria Haw

Mammilloydia Buxb.

NeolloydiaBritton & Rose

ObregoniaFric

OrtegocactusAlexander

PediocactusBritton & Rose

Pelecyphora C.Ehrenb

SclerocactusBritton & Rose

Stenocactus(K.Schum.) A.W.Hill

StrombocactusBritton & Rose

Thelocactus(K.Schum.) Britton & Rose

Turbinicarpus(Backeb.) Buxb. & Backeb.

Tribe Calymmantheae

CalymmanthiumF.Ritter

Tribe Cereae

Cereus repandus, occurs in the Dutch Caribbean, Bolivia and Venezuela.

ArrojadoaBritton & Rose

BrasilicereusBackeb.

Cereus Mill.

SubpilocereusBackeb.

CipocereusF.Ritter

ColeocephalocereusBackeb.

DiscocactusPfeiff.

MelocactusLink & Otto

MicranthocereusBackeb.

PierrebrauniaEsteves

Pilosocereus Byles & G.D.Rowley

PseudopilocereusBuxb.

PraecereusBuxb.

StephanocereusA.Berger

Uebelmannia Buining

Tribe Hylocereeae

DisocactusLindl

EpiphyllumHaw.

Hylocereus(A.Berger) Britton & Rose

Selenicereus(A.Berger) Britton & Rose

PseudorhipsalisBritton & Rose

WeberocereusBritton & Rose

Tribe Notocacteae

Yavia cryptocarpo, only member of its genus, endemic to Argentinian rocky slopes.

Threatened area examples

For example, Echinocereus knippelianus has had its populations greatly decimated by agricultural conversion of lands in Mexico. Its numbers are limited to areas of the State of Nuevo Leon and similar habitats. E. pulchellus and Stenocereus eruca are similar species also threatened by land use conversion in Mexico, especially in coastal areas, where extensive grading has taken place over the last two decades to support tourism and preferred residential development sites for a population of increasing per capita wealth; other pressures in northern Mexico and the southwestern border area of the USA have been created by drug trafficking and illegal immigration routes through the remote desert areas. Within Chile's Elqui Valley, aggressive expansion of fruit tree and vineyard planting have decimated previously rich populations of Copiapoa and Eulychnia cacti.

In the Mojave Desert of Southern California, a number of well-intentioned solar array projects have been proposed that would effectively destroy considerable pristine desert habitat, reducing native populations of numerous cacti species. Arizona's Organ Pipe National Park used to function as an important preserve, but now lacks substantive protection from trampling and trash deposition by illegal immigrants and drug traffickers.

In the southwest USA there are a number of private organizations carrying out important cactus conservation work. For example, ranchers in West Texas have formed the Trans Pecos Heritage Association. Their West Texas lands include such rare cacti as Echinocereus viridiflorus and Coryphantha ramillosa. While the objectives of this group include keeping government interference from private lands, their efforts have resulted in considerable benefits in desert habitat and cacti protection.

Areas of high conservation value which are poorly protected

The Chihuahuan Desert is one of the richest locales for cacti diversity and includes many endangered species. Northern San Luis Potosi is a particularly important area, where 14 different endangered cacti occur in the single locale of Huizache. Other Mexican loci of high density of threatened cacti species include Aramberri, Cuatro Cienegas, Doctor Arroyo, Galeana, Jaumave and Matehuala. The Rio Extorax Basin in Mexico's State of Queretaro is a further locale, which, although disjunct from the Chihuahuan Desert continuum, is circumscribed by the cactus rich locales of Guanajuato, Hidalgo and Queretaro.

The Sonoran Desert is a locale of high conservation value, including the Baja Cape, western Baja coast and the Sea of Cortez islands. The Catavina area is known for its richness in Fouquieria columnaris, Pachycormus discolor, Pachycereus pringlei, Washingtonia robusta and Ferocactus gracilis.